# Epoch

Sometimes referred to as **epoch time**, **POSIX time**, and **Unix time**, **epoch** is an operating system starting point that determines a computer's time and date by counting the ticks from the epoch. Below is a listing of different operating systems and their associated tick and epoch.

Operating System | Tick | Epoch |
---|---|---|

Apple macOS | 1 sec | January 1, 1904 to February 6, 2040. |

Microsoft DOS | 1 sec | January 1, 1980 to January 1, 2108. |

Microsoft Windows | 100 ns | January 1, 1601 to AD 30828. |

OpenVMS | 100 ns | November 17, 1858 to AD 31086. |

POSIX | 1 sec | January 1, 1970 to January 19, 2038. |

S/390 | 244.14 picosec | January 1, 1970 to January 19, 2038. |

Unix | 1 sec | January 1, 1970 to January 19, 2038. |

If you want to figure out the difference between two epoch values, do the following. Use the table below to determine how many hours, seconds, days difference there is by subtracting the two epoch values and then dividing by equivalent number of seconds. For example, if the difference between the two was 14,400, dividing that by 3600 shows the difference is 4 hours.

Time period | Epoch second equivalent |
---|---|

1 minute | 60 |

1 hour | 3600 |

1 day | 86400 |

1 week | 604800 |

2 weeks | 1209600 |

3 weeks | 1814400 |

1 month | 2419200 |

6 months | 14515200 |

1 year | 29030400 |

1 decade | 290304000 |

## Why is an epoch used instead of a date?

Converting a date and time into an epoch value makes it easier to find the difference, add, and subtract from a time value. For example, you could convert the time to an epoch and subtract it from another epoch value to quickly determine the difference. If the difference was 176,400 and you used the above chart or a math formula to find the difference is 2 days and 1 hour (86400 + 86400 + 3600 = 176,400).

## Example of converting an epoch

Many programming languages and some programs have functions that help convert an epoch. If you have an epoch value that you need to convert, you can also use an online epoch converter to convert the values into a readable date.

Julian, Measurement, Operating system terms, Programming terms, Tick, Time